1. Field of the Invention
The invention generally relates to an apparatus and method for the production of ophthalmic lenses. More particularly, the present invention relates to the treatment of a mold which produces an ophthalmic lens, such as a contact lens, to minimize damage to the lens formed within the mold.
2. Description of the Related Art
Contact lenses can be manufactured economically in large numbers by the so-called mold or full-mold process. In the molding process, an ophthalmic lens is manufactured into a final form between typically male and female mold shapes, with no additional finishing work on the surface of the lens or the edges of the lens. Known ophthalmic lens-molding processes are described in, for example, PCT patent application no. WO/87/04390 or in EP-A 0 367 513.
A contact lens produced from a male-female molding process typically has little mechanical stability and a water content of more than 60% by weight. The geometry of the ophthalmic lens is specifically defined by the cavity between the male and female mold halves. The geometry of the edge of the ophthalmic lens is defined by the contour of the two mold halves in the area where they make contact.
In the manufacture a polymer contact lens, a predetermined amount of a semi-viscous polymeric material is placed in the female mold half and the mold is closed by placing the male mold half proximately to the female mold half to create a cavity having a desired size and cavity for a contact lens. Normally, a surplus of polymeric material is used so that when the male and female halves of the mold are closed, the excess amount of the material is expelled out into an overflow area adjacent to the mold cavity. The polymer material remaining within the mold is polymerized or cross-linked with the delivery of radiation thereto through UV light, heat action, or another non-thermal methods.
In U.S. Pat. No. 5,508,317, a new contact lens material is described as representing an improvement in the chemistry of polymerizable starting material for the manufacture of contact lenses. The ‘317 patent discloses a water-soluble composition of a prepolymer which is filled into the mold cavity and then the composition is photochemically cross-linked. The prepolymer has several cross-linkable groups whereby the cross-linking is sufficient to create a finished lens of optical quality within a few seconds without the necessity for subsequent extraction or finishing steps to the contact lens. The improved cross-linking chemistry of the starting polymeric material of the ‘317 patent permits the manufacture of contact lenses at considerably lower cost than the hitherto methods which thus makes it possible to produce disposable contact lenses that are discarded by the user after a single use.
Another process of using the prepolymer to manufacture a contact lens is disclosed in EP-A-0 637 490 which describes a manufacturing process for contact lenses with the prepolymer described in the ‘317 patent. In the ‘490 reference, the polymeric material is filled into a mold comprising two halves placed in proximate non-touching relation to create a thin circular gap between the halves. The circular gap is linked to the mold cavity such that surplus lens material can flow away from the mold cavity into the gap.
The molds for the ophthalmic lenses can be for one time use or reusable. Because of the water-soluble nature of the polymeric material, after the ophthalmic lens has been produced in the mold, the non-crosslinked prepolymer and other residues can be removed rapidly and effectively with water, and the molds air-dried. The cross-linking of the prepolymer is promoted through exposure of the prepolymer to radiation, typically with UV light, and the radiation is typically restricted to the mold cavity by a chromium screen. Through this common production method, only the prepolymer material in the mold cavity is cross-linked to produce the edges of the contact lens with two mold halves closed. The non-crosslinked prepolymer solution, i.e. the prepolymer not exposed to the radiation, is washed away from the shaped and formed contact lens, i.e. cross-linked prepolymer, with water.
There are several methods known in the art to include surfactants and other lubricants to mold surfaces to facilitate release of the contact lens and minimize any damage to the formed lens. An example of the use of surfactants in the molds is U.S. Pat. No. 5,542,978, which discloses a method and apparatus for employing a surfactant in order to assist in the release from each other of mold components of a multi-part mold employed in the molding of polymeric articles, such as a hydrophilic contact lenses, upon completion of the molding process. The surfactant is applied in the form of a film or coating on surface portions of one of the mold components to facilitate both the disengagement between the mold components during demolding and the removal of excess polymeric molding material adhesively deposited on mold surfaces.
In addition, WIPO application WO 00/76738 relates to the preparation of molds for the production of contact lenses. The patent teaches the use of a hard organic coating for the purpose of protecting a disposable plastic mould from attach by monomers used in the lense polymer formulation. The present invention differs, because it claims a plasma treatment for reusable moulds instead of disposable moulds and uses lenses materials not having destructive monomers.
Several problems arise when plastic moulds are used to make contact lenses. One problem is called “bubbling”. Bubbling occurs when the polymer is dispensed into the mould—if the surface of the mould is not correctly treated, bubbles may form between the polymer and the mould surface. The bubbles remain after the lens is cured, resulting in a defective lens. Bubbles can form at the interface between the mold surface and the lens material when the lens materials do not have a good wettability or bubbles can be present in the lens materials during the lens material preparation. When the mold surface has a good wettability to the lens material, the lens material spreads over the mold surface evenly, which can eliminate the possibility of bubble being formed and trapped in the interface.
A lens made from an improperly treated plastic mould can also result in “sticking” to the polypropylene package which contains the lens for autoclaving and ultimate sale to the consumer. Sticking means that the lens adheres to the inner surface of the container where the lens contacts the container, requiring the consumer to apply excessive force to remove the lens therefrom. The excessive force can result in damage to the lens. The surface of the formed ophthalmic lens can depend on the surface of the mold to which the lens material is formed. In other words, the hydrophobic surface tends to make the ophthalmic lens surface rather hydrophobic, which makes good contact to the packaging material, in this case, polypropylene. In other case, if the air (or bubbles) are present at the interface, the surface of the lens may be less cured leaving the surface groups rather mobile. This can contribute to sticking to the polypropylene packaging surface. So, if the mold surface is a right selection to the lens material, the interface can be rather simple, i.e., mold and lens material. The lens surface will not have active mobility at the surface. Finally, if the plastic mould is intended to be reusable, the coating must be durable enough to withstand repeated manufacturing cycles
Accordingly, it would be advantageous to provide an apparatus and method for producing ophthalmic lenses in a reusable mold that does not produce lenses having bubbles or sticking or which allows the production of a large number of commercial grade contact lenses from reusable plastic moulds. It is to the provision of such an improved apparatus and method for the production of ophthalmic lenses, and specifically contact lenses, that the present invention is primarily directed.